1. Field of the Invention
The field of the invention is that of display devices projecting an image on a semi-transparent screen. These devices can be used to display an image facing the user in superimposition on the exterior landscape. One of the possible fields of use is the presentation of information in vehicles, and more precisely in aircraft cockpits, particularly those which have large glazed surfaces.
2. Description of the Prior Art
These display systems enable the user's attention to be kept on his surrounding environment, while providing him with instantaneous access to additional information. In the case of vehicle driving, having information is displayed in the visual field of the external environment means that the user avoids having to look for this information on screens which are conventionally located on his dashboard or instrument panel.
There are various technical solutions for projecting an image on a semi-transparent screen. The most commonly used is shown in FIG. 1. It consists in projecting the image emerging from a projector P emitting in the visible spectrum on to the surface of a transparent diffusing film E. The diffuser diffuses the projected image towards the eye Y of the observer, while remaining relatively transparent, thus providing a view of the exterior. The compromise between transmission and diffusion is not easily found. Diffusing films are used for the projection of advertising on to glazed walls of shops. However, the technique is mainly used at night, that is to say at low ambient light levels. In the daytime, the results of this technique are mediocre. This is because, as shown in FIG. 2, the film E diffuses the sunlight S in all directions (shown by striped straight arrows in FIG. 2). The transmission of the exterior light is represented by a sequence of chevrons in this figure. In these conditions, the film appears milky, the transparency is limited, and the performance of the projected image in terms of reflection and luminosity is poor and unsatisfactory.
The present applicant has therefore proposed, in the application FR 2 986 624 entitled “Optical projector with semi-transparent projection screen”, a display screen including a plurality of light-diffusing patterns regularly distributed over a transparent surface, these patterns possibly including a reflective treatment. By comparison with the preceding systems, the semi-transparent screen of this projector has high transparency combined with high luminosity of the projected image, while also having limited diffraction.
However, the light rays emerging from the projector and diffused by the screen all have a known angle of incidence. Similarly, the solar illumination can only come from certain directions, as certain parts of the cockpit form a natural mask. The prior screen is isotropic. It does not make a fine allowance for the variations in the angle of incidence of the light rays from the projector so as to optimize the performance in terms of diffusion towards the observer's eye.